Current Research on Induced Motion

Induced motion

Induced motion is the altered perceived velocity/direction of target motion by background motion (Duncker, 1929). Duncker moved a cardboard as a background behind a static disk. The disk seemed to move in the opposite direction. Objective target velocity seemed to increase with opposite background motion, but to decrease with background motion in the same objective direction.

Demo

The demo by Holger Kälberer is in Java. If it does not run smoothly, you may try whether closing of other applications improves the presentation.

A red X as a target objectively circles clockwise. Backgrounds are made of black X-distractors. These circle counter-clockwise.

1. Choose 0, 1, 3, or 15 distractors. 0 or 1 distractor make for no motion background, or hardly any background. 3 or 15 distractors induce motion of the red X.
2. Choose a distractor start position in degrees of arc (from 0° to 360°). The red X starts from vertex motion-path position. By chosing a ° value, you shift the relative distractor start position(s). Examples: 0° means distractor(s) start at vertex position(s), too; 180° means distractor(s) start at the troughs of their motion path(s).
3. Press "Start" for the demo. For a halt press "Stop".


4. For variations, reiterate from 1. with different options.
5. You can also choose between a continuous and an intermittent presentation mode.
6. Press "Display hubs" to verify that red Xs move objectively circular even with background motion on.

What do you see?

The motion path of the red X appears ellipse-shaped with background motion of 3 or 15 distractors on. The orientation of the ellipse-shaped path in the fronto-parallel plane shifts with the distractor-start position(s). For instance, with 0°, it appears vertically suppressed but horizontally stretched; with 180°, it appears horizontally suppressed but vertically stretched. The illusion is much weaker with less or no distractors, and with the hubs displayed.

Explanation

Shifting distractor relative to the red X's start position, shifts minima and maxima of directional differences δ between the red X's and the background's motion. Example: With 0°, δ is maximal if the red X is at its vertex and trough motion-path positions, i.e., when it objectively moves horizontally: The red X moves left or right, the opposite directed background motion induces high velocity of the red X, and the red X seems to cover a large horizontal distance. Likewise, with 0°, δ is minimal if the red X is at its extreme right and left motion-path positions, i.e., when it objectively moves vertically: The red X moves downward or upward, the similarly directed background motion induces low velocity of the red X, and the red X seems to cover a minor vertical distance. In effect, the ellipse-shaped motion path of the red X appears horizontally stretched and vertically compressed.

By contrast, with 180°, δ is minimal if the red X is at its vertex and trough motion-path positions, i.e., when it objectively moves horizontally: The red X moves left or right, the similarly directed background motion induces low velocity of the red X, and the red X seems to cover a minor horizontal distance. Likewise, with 0°, δ is maximal if the red X is at its extreme right and left motion-path positions, i.e., when it objectively moves vertically: The red X moves downward or upward, the opposite directed background motion induces high velocity of the red X, and the red X seems to cover a large vertical distance. In effect, the ellipse-shaped motion path of the red X appears vertically stretched and horizontally compressed. For a detailed explanation click here (manuscript in pdf format).